[NOVICE] Setting a DEFAULT when NULL is inserted

Hi List,
    I want a column to get a default value of now() (the timestamp) when a row is inserted. But it's not working because the application that's doing the insertion appears to be putting a null value in, and DEFAULT isn't converting this to now().
   
    Example:
    I have a table with a simple definition:
   
    CREATE TABLE my_table
(
id integer,
insertion_datetime timestamp DEFAULT now()
);

If I do:
insert into my_table (id) values (1);

Then the insertion_datetime gets a value of now() correctly.

But if I do this:
insert into my_table (id, insertion_datetime) values (1, null);

Then the insertion_datetime gets a value of NULL rather than the desired now().

I can see why this happens (I have explicitly told it to put NULL in there after all), but it's not the desired behaviour. I can't change the application, so how do I get any inserted NULL values to become the DEFAULT now()? Am I going to need to use a trigger?

Thanks,
Jonathan